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Percentage of employer matching contribution on additional participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

No definition available.

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Data Type:	dtr-types:percentItemType
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Percentage of participant compensation representing the minimum amount elected by the participant that is eligible for employer matching contribution for participant in employee stock purchase, savings, or similar plan (employee benefit plan (EBP)).

No definition available.

Name:	osbc_EmployeeBenefitPlanEmployerContributionMinimumParticipantCompensationElectedPercentage
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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of participant compensation, equal to or greater than 5 percent, that is elected by the participant and is eligible for employer matching contribution for participant in employee stock purchase, savings, or similar plan (employee benefit plan (EBP)).

No definition available.

Name:	osbc_EmployeeBenefitPlanEmployerContributionParticipantCompensation5OrMoreElectedMatchedPercentage
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Data Type:	dtr-types:percentItemType
Balance Type:	na
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Period of service required for a participant to become eligible for employer matching contributions in an employee stock purchase, savings, or similar plan (employee benefit plan (EBP)).

No definition available.

Name:	osbc_EmployeeBenefitPlanMatchingContributionEligibilityPeriodOfService
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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Term of participant loan of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

No definition available.

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Age at which a participant becomes eligible to receive in-service distributions from an employee stock purchase, savings, or similar plan (employee benefit plan (EBP)).

No definition available.

Name:	osbc_EmployeeBenefitPlanParticipantAgeEligibilityToReceiveInServiceDistributions
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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Amount of employer profit-sharing contributions allocated to participants in an employee stock purchase, savings, or similar plan (employee benefit plan (EBP)).

No definition available.

Name:	osbc_EmployeeBenefitPlanProfitSharingContributionsAmount
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Number of years of service required for a participant to become eligible for profit-sharing contributions in an employee stock purchase, savings, or similar plan (employee benefit plan (EBP)).

No definition available.

Name:	osbc_EmployeeBenefitPlanProfitSharingContributionsEligibilityNumberOfYearsOfService
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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Represents the percentage of vesting.

No definition available.

Name:	osbc_EmployeeBenefitPlanVestingPercentage
Namespace Prefix:	osbc_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Amount of increase in net asset available for benefit from cash and noncash contributions from employer of participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-05(a)(Schedule III))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 50
-Paragraph 2B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477783/962-325-50-2B

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 45
-Paragraph 7
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477063/962-205-45-7

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(4)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(4)(a)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

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Data Type:	xbrli:stringItemType
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Period Type:	duration

Percentage of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Percentage of participant compensation eligible for employer matching contribution for participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Amount of decrease in forfeited nonvested account of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) from employer contribution.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478873/962-325-55-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-04(5)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

Name:	us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingAmount
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Percentage of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow to total participant account balance.